1. Field of the Invention
The present invention generally relates to the cancellation of the anti-resonance signal from a resonator and more particularly to a method and apparatus for cancelling the anti-resonance in micro-mechanical, surface acoustic wave (SAW) and bulk acoustic wave (BAW) resonators.
2. Description of the Background Art
Cellular telephone systems are now used in many countries in the world and have often replaced traditional wired systems. However, in different countries, the specific standards that have been adopted are often different. Thus, cellular devices from one country are not operable within the system in another country. The result is that manufacturers must produce completely different handsets for different countries, which requires the engineering and design of completely different items, which is more expensive than having a single common item which is usable in all countries. One method that has been suggested to overcome this problem is a software-defined radio which utilizes an entire band of RF or IF signal with the particular channel being selected using a programmable digital filter. However, this requires an A/D converter which has a speed in the GHz range and also has a dynamic range over 100 dB. One type of converter which shows promise in performing within these parameters is a signal-delta A/D converter which includes a sigma-delta modulator and a digital filter. An example of such a system is shown in FIG. 1 of related U.S. patent application Ser. No. 10/188,071. The modulator shown there includes a resonator which provides noise shaping.
Communication systems have widely used SAW resonators due to its high Q factor which cannot be achieved by active filters. Recently developed IC compatible micro-mechanical resonators have been used to replace the bulky SAW resonators to further reduce the volume of the system. However, the resonant frequency of micro-mechanical resonators is often limited to hundreds of MHz. However, recent advances in bulk acoustic wave technology has made it possible to use BAW resonators in conventional CMOS technology. These BAW resonators have a higher resonant frequency and are typically in the GHz range.
In all three types of these resonators, there are two modes of resonance, namely the series mode and the parallel mode. In the series mode, the impedance of the resonator is at a minimum. In the parallel mode, the impedance of the resonator is at a maximum. The frequency at which the series resonance occurs is referred to as the resonant frequency while the frequency at which the parallel resonance mode occurs is called the anti-resonant frequency.
FIG. 1 is a graph showing the relationship between the frequency of the resonator and the admittance (inverse of impedance). A maximum point is seen at 1 which is the series resonance mode. This occurs at frequency fr. A minimum point 2 is the parallel resonance mode which occurs at the anti-resonance frequency fa.
FIG. 2 shows a typical equivalent circuit for a resonator. It includes a static capacitance Cp, 3 which is connected in parallel to a series circuit of resistor Rm, 6, capacitor Cm, 5 and inductor Lm, 4. The static capacitance 3 alters the transfer function of the ideal resonator by introducing two transmission zeros. In different applications, the presence of the anti-resonance may be desirable, such as in filtering and making oscillators. However, in other situations, it is not desirable such as in the sigma-delta modulator of U.S. patent application Ser. No. 10/188,071 discussed above. The presence of anti-resonance makes it difficult to realize the transfer function of the modulator and therefore it is desirable that it be cancelled. Likewise, in other modulators and in other situations, it may be desirable to cancel the anti-resonance in order that the resonance peak is emphasized.